In general, serial recording systems for printing data while reciprocating in a direction perpendicular to the feed direction of a printing medium such as a sheet are widely adopted in terms of low cost and easy downsizing in printing apparatuses for printing information such as desired characters or images on a sheet-like printing medium such as a sheet or film, as the information output apparatuses of a wordprocessor, personal computer, and facsimile.
The structure of a printhead used in such a printing apparatus will be described by exemplifying an inkjet printhead for printing data by using thermal energy. The inkjet printhead has, as printing elements, heating elements (heaters) disposed at portions communicating with orifices (nozzles) for discharging ink droplets. A current is supplied to the heating elements to generate heat and form bubbles in the ink to discharge ink droplets, thereby printing data. This printhead enables laying out many orifices and heating elements (heaters) at high density, and can realize a fine printing image.
To print data by this printhead at high speed, it is desirable to simultaneously drive many heaters. However, the number of simultaneously drivable heaters is limited due to restriction on the power supply ability of a power supply and a voltage drop caused by the parasitic resistance of wiring. For this reason, the maximum current value is suppressed by dividing a heater array into groups each including a plurality of heaters and time-divisionally driving heaters within a group.
A circuit arrangement for this driving is disclosed in Japanese Patent Laid-Open No. 9-327914. FIG. 16 is a circuit diagram showing an arrangement of 128 heaters and their driving circuit.
In FIG. 16, reference-symbols H1 to H128 denote heaters as printing elements; and T1 to T128, transistors for driving the respective heaters. Reference numeral 600 denotes a 4 to 16 decoder for decoding block control signals B1, B2, B3, and B4 supplied from a printer main body and generating block selection signals N1, N2, . . . , N16; 603, an 8-bit shift register for serially receiving printing data DATA in accordance with a clock signal CK supplied from the printer main body; 604, an 8-bit latch circuit for latching 8-bit printing data DATA stored in the 8-bit shift register 603 in accordance with a latch signal LATCH supplied from the printer; and 605, an AND circuit for deriving a logical product of an enable signal ENB and the bits of the 8-bit data latched by the 8-bit latch circuit 604.
Outputs from the AND circuit 605 are supplied as printing signals D1 to D8 to the heating elements. The driving timings and pulse widths of the heating elements are determined by the outputs from the AND circuit 605 and the block selection signals N1 to N16 as outputs from the 4 to 16 decoder 600. When the enable signal ENB is “High”, the heating elements are driven.
FIG. 17 is a timing chart showing the states of signals concerning driving of the printhead in the arrangement shown in FIG. 16. According to this timing chart, the serial transfer timing of printing data to the 8-bit shift register 603 does not overlap the driving timing of the heating element.
In many cases, printing elements (heaters) and their driving circuit are formed on one substrate by a semiconductor manufacturing technique in order to achieve high integration degree.
FIG. 18 shows a layout in which the circuit of FIG. 16 is formed on a printing element substrate. In FIG. 18, reference numeral 801 denotes an ink supply opening which allows ink to pass from the lower surface of the substrate and to be supplied onto the upper surface of the substrate. Two systems of circuits in FIG. 16 are symmetrically arranged on the two sides of the ink supply opening 801 formed at the center of the substrate. Heaters and transistors are laid out in the longitudinal direction of the ink supply opening 801. Decoders 600, shift registers 603, and latch circuits 604 are disposed on the two sides in a direction along which heater arrays extend. Signal lines from the decoders and shift registers to the heaters are laid out parallel to the heater array direction.
To meet demands for high printing speed and high resolution, the numbers of nozzles and heaters of the printhead increase, which poses the following problem with the printing element substrate.
As the number of heaters increases, the number of signal lines for selecting the heaters also increases. As for the shape of the printing element substrate, the dimension of the board in the heater array direction increases along with an increase in the number of heaters. The dimension of the board in a direction perpendicular to the heater array direction also increases because of wiring lines connected to the heater driving circuit. As a result, the board area greatly increases.
When a board is formed on a wafer by a semiconductor manufacturing technique, an increase in board area decreases the number of boards per wafer and yield, thereby extremely increasing the cost of the printing element substrate.
As the dimension of the board in the heater array direction increases along with an increase in the number of nozzles, wiring lengths from the decoder and shift register also increase. This causes signal delays between the input and the heater driving circuit, inhibiting high-speed driving. Further, the printhead is readily influenced by external noise and may malfunction.